Processes for preparing polycarbonates are known in the art. Generally a dihydric phenol, such as bisphenol A is reacted with phosgene with the use of optional mono-functional compounds as chain terminators and tri-functional or higher functional compounds as branching or crosslinking agents. Reactive acyl halides are also condensation polymerizable and have been used in polycarbonates as terminating compounds (mono-functional), comonomers (di-functional) or branching agents (tri-functional or higher).
U.S. Pat. No. 4,367,186 disclose a process for producing cross-linked polycarbonates wherein a cross-linkable polycarbonate contains methacrylic acid chloride as a chain terminator. A mixture of bisphenol A, aqueous sodium hydroxide and methylene chloride is prepared. To this is added a solution of methacrylic acid chloride in methylene chloride. Then, phosgene is added and an additional amount of aqueous sodium hydroxide are added to keep the pH between 13 and 14. Finally, the triethylamine coupling catalyst is added. Unfortunately, in a process such as this where an acid chloride is added to the reaction mixture when it is at a high pH, a major portion of the acid chloride hydrolyzes to the non-reactive acid form under the alkaline reaction conditions. Furthermore, a portion of the methacrylic acid chloride reacts with bisphenol A (two moles of phosgene, one mole of bisphenol A, two moles acid chloride) to form low weight molecular weight dicarbonates which have a negative impact on the mechanical properties of the produced polycarbonate.
EP 273 144 discloses a branched poly(ester)carbonate which is end capped with a reactive structure of the formula --C(O)--CH.dbd.CH--R, wherein R is hydrogen or C.sub.1-3 -alkyl. This polycarbonate is prepared in a conventional manner using a branching agent, such as trimellityl trichloride and an acryloyl chloride to provide the reactive end groups. According to the examples the process is carried out by mixing water, methylene chloride, triethylamine, bisphenol A and optionally para-t-butyl phenol as a chain terminating agent. The pH is maintained at 9 to 10 by addition of aqueous sodium hydroxide. A mixture of terephthaloyl dichloride, isophthaloyl dichloride, methylene chloride, and optionally acryloyl chloride and trimellityl trichloride is added dropwise. Phosgene is then introduced slowly into the reaction mixture. While this procedure may be useful on laboratory scale, it is not suitable for large scale continuous production.
U.S. Pat. No. 5,171,824 discloses a process for the preparation of arylcyclobutene terminated condensation polymers wherein an arylcyclobutene chain terminating compound is added to a condensation polymerization process along with a multihydric compound, such as bisphenol A, and a condensation polymer precursor, such as phosgene, a haloformate or a carbonate ester. The condensation polymerization is conducted at a pH of 12.5. Unfortunately, a major portion of the arylcyclobutene chain terminating compound, such as benzocyclobutene-4-carbonyl chloride, hydrolyzes under these reaction conditions.
Randomly branched polycarbonates and methods of preparing them are known from U.S. Pat. No. 4,001,184. At least 20 weight percent of a stoichiometric quantity of a carbonate precursor, such as an acyl halide or a haloformate, is reacted with a mixture of a dihydric phenol and at least 0.05 mole percent of a polyfunctional aromatic compound in a medium of water and a solvent for the polycarbonate. The medium contains at least 1.2 mole percent of a polymerization catalyst. Sufficient alkali metal hydroxide is added to the reaction medium to maintain a pH range of 3 to 6 and then sufficient alkali metal hydroxide is added to raise the pH to at least 9 but less than 12 while reacting the remaining carbonate precursor. This procedure is very suitable for batch processes which allow the use of pH electrodes and close control of the pH in the reaction mixture. Unfortunately, this procedure is less suitable in a continuous production process.
U.S. Pat. No. 5,142,088 discusses the disadvantages of branched polycarbonates which are produced according to a conventional process wherein phosgene undergoes reaction with a bisphenol in the presence of a polycarboxylic acid or a derivative thereof, preferably trimellitic acid trichloride, under alkaline conditions typically involving a pH above 10. One of the disadvantages of the conventional process is the formation of diaryl carbonates. Diaryl carbonates are formed as a result of the reaction of phosgene with the monohydric phenol used as a chain terminator. Diaryl carbonates cause numerous problems in polycarbonate molding operations, such as difficulties in removing molded polycarbonate articles from the mold. U.S. Pat. No. 5,142,088 discusses various patents which disclose methods for preparing polycarbonates from chloroformate oligomers, typically comprising mono- and/or bis-chloroformates. Such methods decrease the proportion of diaryl carbonate in the polycarbonate products because the monohydric phenol employed as chain terminating agent is not introduced into the reaction mixture until the chloroformate composition has been prepared and unreacted phosgene has been purged from the system. Also see EP 369 422 and WO 94/18258 which teach a method to reduce diaryl carbonate formation by withholding addition of the chain terminating agent to a polycarbonate reaction mixture until the reaction of a bisphenol with phosgene has substantially been completed.
However, U.S. Pat. No. 5,142,088 teaches that incorporation of branching agents, such as trimellitic acid trichloride, in polycarbonate-forming reactions from chloroformate oligomers has generally not proved successfil. The branching reaction between a phenolic species and the mentioned branching agent requires a pH in the range from 10 to 12 whereas the chloroformate-forming reaction requires a lower pH, at which incorporation of the branching agent is incomplete. U.S. Pat. No. 5,142,088 indicates that little if any branching results if the above-mentioned branching agent is introduced during conversion of the chloroformate to polycarbonate. In order to solve the problem of producing relatively diaryl carbonate-free branched polycarbonates, U.S. Pat. No. 5,142,088 suggests a process wherein in step (1) an aromatic poly(acyl halide) is reacted with a dihydric phenol in the presence of a catalyst in a reaction system of water, having a pH of 10-12, and a water-insoluble organic system to form an ester polyphenol, in step (2) the pH of the reaction system is reduced to a value from 7 to 10 and phosgene is introduced into the system, and in step (3) a catalyst is added and the pH is raised to value between 10 to 14 to form the polycarbonate. Unfortunately, this process is only suitable for batch operation where the use of pH electrodes and variation of the pH value is possible.